Optical receivers used in communications systems generally comprise a photo-detector 2 for converting an optical signal 4 into an electrical signal 6 (See FIG. 1a). The electrical signal 6 output by the photodetector 2 is supplied to a linear channel 8 that may consist of a high gain amplifier and a low pass filter, and a Clock and Data Recovery (CDR) circuit 10.
The most extensively used photo-detectors in fiber optical systems are the P-Intrinsic-N (PIN) photodiode and the avalanche photodiode (APD). Optical receivers that use an APD normally provide higher sensitivity than that use PIN photodiode, since APDs have internal gain from the optical-to-electrical conversion process.
The performance of an APD is typically characterised by various performance parameters such as its gain, noise and bandwidth. These parameters vary with operating conditions (primarily temperature and optical input power) and are also subject to random manufacturing variations. As a result, each APD is unique, and exhibits a uniquely different response to variations in the input optical power, reverse bias voltage and temperature.
For example, when an APD is operated below its reverse breakdown voltage, an increase in the reverse bias voltage results in amplification. This is the region of normal APD operation. However at a reverse bias voltage equal to the breakdown voltage, dark currents increase exponentially, causing the receiver to be saturated with noise and possibly damaging or destroying the APD. Therefore the reverse bias voltage is normally set at a specified number of volts below the breakdown voltage specified by the manufacturer. However since each APD is unique, the breakdown voltage of each APD is different, and thus a different reverse bias voltage must be found for each APD.
As a result, careful control of the reverse bias voltage is required to maintain acceptable APD performance. This control function is typically provided by an APD bias controller 12 which comprises a micro-controller 14, a memory 16 (such as an EEPROM), one or more digital-to-analog converters (DACS) 18, and one or more analog-to-digital (ADCs) 20. A voltage converter 22 may be inserted between the controller 12 and the APD 2 to convert the DAC 18 output voltage 24 to the appropriate bias voltage 26 needed to drive the APD 2. A Thermal Electric Cooler (TEC) 23 may also be used to control the temperature of the APD 2, and thereby mitigate APD performance variations due to temperature fluctuations. Characteristic data for the APD 2 is stored in the memory 16, and used by the micro-controller 14 to determine the appropriate bias voltage. Various techniques are known for accomplishing this.
For example, U.S. Pat. No. 5,953,690, which issued to Lemon et al. on Sep. 14, 1999 teaches an intelligent fiber-optic receiver. During calibration procedures for the receiver, a thermal chamber is used to enable characterization of the APD (and its supporting control and monitoring circuits) over a defined operating temperature range. Characteristic data and/or curves defining these operational control and monitoring functions, over the range of operating conditions (e.g. temperature, input optical power etc.), are stored in non-volatile memory such as EEPROM. During operation, an embedded microcontroller detects current operating conditions of the APD, and uses this information to access the stored data for controlling the bias voltage.
U.S. Pat. No. 6,313,459, which issued to Hoffe et al. on Nov. 6, 2001, teaches an operational algorithm, and calibration process, for an APD receiver which takes into account an APD behavioural model. In-situ optical and electrical measurements of the APD are performed to determine key constants for use in the model.
U.S. Pat. No. 6,222,660, which issued to Traa on Apr. 24, 2001, teaches an adaptive power supply for an avalanche photodiode (APD). In cases where an optical input signal is not present, the adaptive power supply applies a swept voltage to the APD while monitoring the photodiode current. When breakdown occurs, the voltage is noted and the bias voltage from the adaptive power supply is set at a specified offset below the breakdown voltage. In cases where a source of optical digital data signal is present, it is coupled to the input of the APD via a programmable optical attenuator. The electrical digital signal output from the APD is input to a bit error rate counter, the output of which is monitored. For different input optical power levels, the APD bias voltage is swept by the adaptive power supply, so as to determine a constant optical power level curve over which the bit error rate is virtually zero. This is repeated for a plurality of optical power levels, the resulting family of curves defining a region within which the bit error rate is virtually zero. During operation, the adaptive power supply is set to a value that falls within the “virtually zero” bit error rate region for the expected optical power level input.
All of these approaches suffer a limitation in that extensive measurements are required in order to characterize the APD. These measurements must necessarily be conducted separately for each APD, can be time consuming, and cannot be conducted when the receiver is receiving “live” optical signal traffic. This also means that updating the characteristic data to compensate for age-related drift of component parameters, for example, may be difficult and/or expensive to implement. Furthermore, during operation, the bias voltage is controlled based on local parameters (i.e. optical power input, APD temperature etc.) and the stored characteristic data in an effort to optimize performance of the APD. However, this functionality may not succeed in optimizing performance of the receiver as a whole.
Accordingly, cost effective dynamic control the bias voltage of a photodide remains highly desirable.